The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty.
The field of the invention is noise minimization for magnetic resonance imaging and more specifically to delay based active noise cancellation for magnetic resonance imaging.
Magnetic resonance imagers (MRI) are extremely valuable tools for medical scanning. MRI systems are able to identify different types of tissue within the human body by placing the tissue in a strong magnetic field generated by a superconducting magnet and exciting the tissue (specifically the spin states of the molecules within the tissue) with a pulse of RF energy, and then measuring the electromagnetic emissions of the tissue as its spin states relax to the rest position. The MRI scanner is able to map the spatial locations of the different types of tissue by creating gradients in the magnetic field that change the response of the tissue as a function of spatial location within the body.
Researchers have long recognized the severity of the noise problem in MRI scanners and that they impair patient comfort and contribute to patient anxiety. Numerous studies have examined the acoustic characteristics of these noise fields. The MRI sound is created by xe2x80x9cknockingxe2x80x9d of the gradient coils. The gradient coils in the MRI are oriented orthogonally to the field of the static magnet. During the imaging, the current in these coils is turned on and off at 5-10 ms intervals. Each time the coil current is switched on in the magnetic field, a force is created that is orthogonal to both the direction of the current and the magnetic field (i.e. radially outward). This sudden force on the gradient coil causes it to xe2x80x9cknockxe2x80x9d against its mounting, creating acoustic noise. The characteristics of the noise are therefore directly related to the gradient pulse sequence used in the scan, and will vary substantially according to the scan performed. Studies have examined the acoustic characteristics of the noise fields by placing a condenser microphone inside the MRI. These studies, after verifying that the microphone was not effected by the electromagnetic fields in the scanner, have generally shown that the imager noise in a 1.5 Telsa MRI scanner is approximately 100-105 dB SPL during a relatively noisy MRI scan. More recent studies indicate that the peak noise level in a functional MRI scanner is approximately 118 decibel for a 1.5 Telsa MRI and 134 decibel for a 3.0 Telsa MRI scanner, suggesting that the more powerful MRI systems may be 15 decibel louder than the 1.5 Telsa scanners.
The noise generated by xe2x80x9cgradient coil knockingxe2x80x9d are disadvantageous for several reasons. First, the noise levels are sufficiently high that they could damage the patient""s hearing. Occupational Safety and Health Agency guidelines limit occupational exposure to noise to 30 minutes per day at 105 decibels SPL and to less than 15 minutes per day at 115 decibel SPL. Since MRI scans can take 40 minutes or longer, patients who are scanned without hearing protection may exceed these limitations (particularly in the 3.0 Telsa scanners). The risk of exposure may be even worse in patients taking drugs that enhance the damaging effects of sound, such as aminoglycoside antibiotics. Second, the noise generated by the MRI scanner is annoying to patients and contributes to their anxiety about the MRI scan. It is now common practice to provide patients with pneumatic headphones that attenuate the sound field and mask the imaging noise with music. However, the imaging noise remains clearly audible over this music. Finally, the MRI noise inhibits the use of functional MRI scanners (FMRI) to examine the areas of the brain stimulated by acoustic stimuli. The FMRI compares the activity in the brain with no stimulus to the activity that occurs when the subject is exposed to a stimulus. The fMRI generates such a loud acoustic stimulus that it is impossible to conduct a controlled study of the processing of sound by the brain.
In the past, both passive and active attenuation have been used to mitigate MRI noise. The passive attenuation usually consists either of foam insert ear plugs or ear muffs. Under normal circumstances these systems can provide 20 -30 decibel of attenuation. Commonly these hearing protection devices are combined with a pneumatic-driven system carrying sound to the patient""s ears via plastic tubes (similar to the type of headset once commonly used on commercial aircraft). The pneumatic sound system allows the operator to give verbal instructions to the patient or play music, but probably somewhat reduced the passive attenuation of the hearing protection devices. Recently, a noise-attenuating headset has been developed that uses shielded non-magnetic (piezoelectric) electro-acoustic transducers to generate an audio signal inside the MRI without the considerable delays inherent in pneumatic headphones (as sound propagates from the driver unit into the MRI magnet bore through a tube).
Known active MRI attenuation noise cancellation systems include a system that uses a standard pneumatic headset system in which polyethylene tubes carry an acoustic signal from a pneumatic driver unit outside the MRI system to a pair of earmuffs worn by a patient. The pneumatic headset is modified with two additional polyethylene tubes that carry the sound signal inside the earmuff away from the headset to a pair of electret microphones located inside the MRI room. The electret microphones are connected to an electro-optical transducer which carry the earmuff noise signal to the shielded control room via fiber optic cables. A xe2x80x9ccompensation amplifierxe2x80x9d produces an anti-noise signal by filtering the noise signal measured by the microphones and playing this signal back through the pneumatic headphones. An average attenuation of 11.1 decibel was reported for this system.
However, there are a number of issues with this MRI noise-cancellation system which are unresolved. The first limitation involves the frequency spectrum where the noise is canceled. The system is only effective at low frequencies (from 40 Hertz to 500 Hertzl and this is inconsistent with measurements of imager noise which indicate that most of the MRI noise occurs around 1 kiloHertz. Indeed, in a T1 pre-scan, 95% of the acoustic power in the noise signal was in the range from 840 Hertz-1920 Hertz. Thus, it does not appear that this prior art MRI system would be capable of attenuating the MRI scans by 10 dBA. The second issue or limitation with the system is the handling of the large time delays between the acoustic noise signal and its measurement by the electret microphones, and between the generation of the acoustic anti-noise signal by the compensation and its arrival at the listener""s ear. Both the measurement and production of sound in the system are limited by the propagation speed of sound. The signal at the patient""s ear must travel down the tube before reaching the electret microphones, a distance of at least 5 ft and a delay of at least 5 ms, and the antinoise signal must travel back down the tube before reaching the listener""s ears, at distance of perhaps 20 ft and a delay of 20 ms. Thus there is a built in delay in the control loop of this system of at least 20 ms, or 10 periods of a 500 Hz signal. Such a delay would compromise most traditional noise canceling algorithms such as the LMS algorithm.
Another known prior art active MRI noise cancellation system uses non-magnetic microphones located at fixed locations inside the magnet bore to record the error signal, and non-magnetic piezoceramic loudspeakers located at fixed locations to generate the noise cancellation signal. Thus, the system cancels the MRI noise signal at fixed locations inside the MRI, rather than at the locations of the patient""s ears. The system uses a two-stage adaptive processing algorithm to generate the noise cancellation signal. The system is capable of reducing acoustic noise by 15-25 dB decibel? We need to be consistent in fast scan sequences and by up to 10 dB in slower xe2x80x9cimpulsivexe2x80x9d scan sequences. However, it should be noted that this cancellation occurs at the locations of the error microphones, and not necessarily at the locations of the patients ears. Consequently, the actual noise attenuation experienced by the patient may be substantially lower.
A third prior art active noise cancellation system includes a feed-forward system in which a reference signal is processed to generate an anti-noise signal played by an electro-acoustic transducer to cancel the MRI noise at the location of error microphones placed inside the MRI near the patients head. The cancellation sound is generated by a loudspeaker located outside the magnet bore and transmitted to the patient""s ears through tubes. Using pulse codes, this known prior art system predicts the acoustic noise generated by the MRI before the noise is actually created. This is a tremendous advantage in the active cancellation of MRI noise for two reasons. First, it allows the noise cancellation system to construct a nearly perfect inverse noise waveform a priori without exposure to the current noise generated by the MRI. The system is reactive, and must wait until the characteristics of the noise change before starting to adapt to the new noise. In traditional adaptive noise cancellation system, each time the noise changes the system will cease canceling the noise effectively until the algorithm has time to adapt to the change. In contrast, an active noise cancellation system that uses a priori information from the pulse code sequence can generate a noise cancellation signal before the noise signal is measured by the system and does not have to adapt to changes in the signal.
However, the prior art MRI active-noise-reduction systems have significant drawbacks. Prior art implementations require extensive modification of the MRI system. For example, microphones and loudspeakers must be permanently mounted inside the MRI system or relatively extensive software changes are required. A noise cancellation system that requires modifications to the MRI system is probably not economically feasible for implementation in the large installed base of MRI scanners because of the extreme expense and fragility of the MRI scanners. The costs of modifying the MRI and ensuring its subsequent functionality would dwarf the costs of the hardware involved in the cancellation system.
Additionally, all of the prior art systems have significant technological drawbacks. One prior art system does a good job of canceling the MRI noise at fixed-location microphones inside the scanner, but these microphone locations may not accurately reflect the noise field at the patients ears. Another prior art system requires a linear or well-characterized non-linear transfer characteristic between the pulse currents entering the gradient coils of the MRI and the resulting acoustic noise. This may be an unreasonable expectation.
The present invention some important advantages over these prior systems. The system described herein is based on a completely novel method of active noise control that is derived directly from the acoustic noise characteristics of the MRI system.
A system and method for actively canceling the acoustic noise generated by changes in the electric current within the gradient coils of a magnetic resonance -imager based on the finding that the acoustic noise in the magnetic resonance imager is highly periodic but that the period of the magnetic resonance imager noise changes substantially during a scan. The acoustic noise signal is measured at the ears of a patient undergoing a magnetic resonance imaging, delayed by a variable number of samples and the resulting signal is subtracted from the acoustic noise signal. Magnetic resonance imaging noise cancellation occurs at the level of 20 decibels or more.
It is therefore an object of the invention to provide an instantaneously adaptable magnetic resonance imaging noise cancellation system.
It is another object of the invention to provide an inherently stable magnetic resonance imaging noise cancellation system.
It is another object of the invention to provide a magnetic resonance imaging noise cancellation system capable of coping with propagation delays.
It is another object of the invention to provide a magnetic resonance imaging system implemented inside an MRI system without modifying the MRI system.
These and other objects of the invention are described in the description, claims and accompanying drawings and are achieved by an improved patient comfort, magnetic resonance imaging noise cancellation system comprising:
a magnetic resonance imaging system, a patient mounted pneumatic headset system;
an error microphone connected to said patient mounted pneumatic headset measuring an acoustic noise signal at an ear of said patient; and
a noise cancellation processor producing an acoustic noise signal canceling waveform, said acoustic noise signal canceling waveform being a delayed and inverted acoustic noise signal output from said error microphone.